The present invention relates to a composition for mechanical chemical polishing of a layer in an insulating material based on a polymer with a low dielectric constant.
Electronic devices made on a silicon slice must be connected to each other by means of interconnecting metallic tracks to constitute the desired electronic circuit. Each metallic interconnecting level must be electrically insulated. To this end, each level is encapsulated in a dielectric layer.
The interconnecting metallic tracks of integrated circuits are most often made by metal reactive ionic etching according the following sequence: an aluminium or aluminium alloy film approximately 10xe2x88x9212 m thick is deposited by means of a beam of electrons or beam of ions (sputtering); the design of the interconnecting circuit is then transferred there by photolithography then by reactive ionic etching (RIE). The tracks thus defined are then covered with a dielectric layer, usually based on silicon oxide, most often obtained by decomposition in the vapour phase of tetraethylorthosilicate (TEOS). This layer is then planarized by mechanical chemical polishing.
In order to reduce the capacitances induced by this dielectric, a route consists of using materials with a low dielectric constant. The best candidates were itemized by N. H. Hendricksxe2x80x94Mat. Res. Soc. Symp. Proc., vol. 443, 1997 p 3-14, and by L. Petersxe2x80x94Semi Conductor Internationalxe2x80x94September 1998, p. 64-74.
The main families retained are:
fluorinated poly(arylethers) such as Flare(copyright),
poly(arylethers) such as PAE-2,
fluorinated polyimides,
hydridosilsesquioxanes and alkylsilsesquioxanes,
bisbenzocyclobutenes such as Cyclotene(copyright),
poly(p-xylylene) such as Parylene(copyright) N and poly(xcex1, xcex1, xcex1xe2x80x2, xcex1xe2x80x2-tetrafluoro-p-xylylene) such as Parylene(copyright) F,
the aromatic ether polymers of perfluorocyclobutane,
aromatic hydrocarbons such as SiLK(copyright).
In order to be integrated into a microelectronic interconnecting process, these polymers with a low dielectric constant must in the main satisfy the following criteria:
filling of trenches of widths greater than 0.35 pm,
a glass transition temperature greater than 400xc2x0 C.,
a low absorption of humidity ( less than 1%),
a high thermal stability ( less than 1%/h at 450xc2x0 C.),
and an excellent adhesion to metallic layers and other dielectric layers.
In addition, the polymer layer deposited must then be able to be planarized by mechanical chemical polishing.
Y. L. Wang et al in Thin Solid Films, (1997), 308-309. p. 550-554, polished a layer of alkylsiloxane using an abrasive solution containing fumed silica particles in a basic medium (pH=10.2), or particles of zirconium oxide in acid medium (pH=3.3-4.6).
G. R. Yang et al. in J. of Electronic Materials, Vol. 26, No. 8, 1997, p.935-940 studied the surface quality of a layer of Parylene(copyright)N obtained after polishing with different abrasive solutions containing alumina particles in an acid or alkaline medium. The polishing speeds obtained were low whatever abrasive solution was used.
J. M. Neirynck et al in Mater. Res. Soc. Symp. Proc., 1995, Vol. 381, p. 229-235, attempted to polish three types of polymers with low dielectric constants, Parylene(copyright), benzocyclobutene (BCB) and a fluorinated polyimide, using abrasive solutions containing alumina particles in basic medium or acid medium.
At present, the integration of layers of polymer with a low dielectric constant into a microelectronic interconnecting process mainly comes up against the inadequate performances obtained on this type of material during planarization by mechanical chemical polishing. This stage is still poorly mastered, in particular as regards the polishing speed and the surface state of the polished surface.
It has been noted in a surprising and unexpected fashion that the use of an acid aqueous suspension of cationized colloidal silica, containing colloidal silica particles not linked to each other by siloxane bonds, allows a layer of insulating material based on a polymer with a low dielectric constant, in particular of SiLK(copyright) type, to be polished, with the result that a very good compromise is obtained regarding the polishing speed, the uniformity of polishing and the surface state of the polished surface.
A subject of the present Application is therefore a composition for mechanical chemical polishing of a layer in an insulating material based on a polymer with a low dielectric constant, characterized in that said polishing composition comprises an acid aqueous suspension of cationized colloidal silica containing individualized colloidal silica particles, not linked to each other by siloxane bonds and water as the suspension medium.
By xe2x80x9cacid aqueous suspension of cationized colloidal silicaxe2x80x9d, is meant an acid aqueous suspension of colloidal silica surface modified by tri- or tetravalent metal oxides such as aluminium, chromium, gallium, titanium or zirconium and which are in particular described in the book xe2x80x9cThe Chemistry of Silicaxe2x80x94R. K. Iler-Wiley Interscience (1979)xe2x80x9d p. 410-411.
Under preferred conditions for implementing the invention, the acid aqueous suspension of cationized colloidal silica containing colloidal silica particles, not linked to each other by siloxane bonds, is obtained from a solution of aluminium hydroxychloride at approximately 50% by weight in which an alkaline silica sol stabilized with sodium and at a pH of about 9 is introduced under agitation. In this way a stable suspension of cationized colloidal silica particles with a pH comprised between 3.5 and 4 is obtained.
In the present Application by xe2x80x9cinsulating material based on a polymer with a low dielectric constant of SiLK(copyright) typexe2x80x9d is meant a material as described by PH. Towsend et al in Mat. Res. Soc. Symp. Proc. 1997, 476, p. 9-17. This material is constituted by an oligomeric solution with a viscosity of 30 centipoises the polymerization of which does not require a catalyst and does not lead to the formation of water. The polymerized network is an aromatic hydrocarbon which does not contain fluorine. Its dielectric constant is 2.65, its glass transition temperature is greater than 490xc2x0 C. and its refractive index is 1.63.
Under preferred conditions for implementing the invention, the aforementioned aqueous suspension of colloidal silica is cationized by tri- or tetravalent metal oxides such as aluminium, chromium, gallium, titanium or zirconium and quite particularly aluminium.
Under other preferred conditions for implementing the invention, the above composition for mechanical chemical polishing will have a pH comprised between 1 and 6, preferably between 2 and 4.
Under yet other preferred conditions for implementing the invention, the above polishing composition contains cationized colloidal silica particles, not linked to each other by siloxane bonds, with a diameter comprised between 3 and 250 nanometers, preferably between 10 and 50 nanometers.
A subject of the present invention is also a process for mechanical chemical polishing of a layer of insulating material based on a polymer with a low dielectric constant in which abrasion of said layer of insulating material is carried out by rubbing said layer using a fabric containing an abrasive, characterized in that the abrasive contains an acid aqueous suspension of cationized colloidal silica containing individualized colloidal silica particles, not linked to each other by siloxane bonds, and water as the suspension medium.
A subject of the present invention is also an abrasive which is useful in particular for mechanical chemical polishing of a layer of insulating material based on a polymer with a low dielectric constant, comprising a fabric impregnated with an acid aqueous suspension of cationized colloidal silica containing individualized colloidal silica particles, not linked to each other by siloxane bonds, with a diameter comprised between 3 and 250 nanometers, with a pH comprised between 1 and 6, and water as the suspension medium.
Preferred conditions for implementing the abrasive compositions described above also apply to the other subjects of the invention envisaged above.